1. Field of the Invention
The present invention relates to an optical recording medium with which the recording, erasing, and reproduction of information can be made by irradiation of light.
More particularly, the present invention relates to rewritable phase change type optical recording media such as optical disks, optical cards, optical tapes, etc. from which it is possible to erase and rewrite information. Such recording media may also record information signals at high speed and in high density.
2. Description of the Prior Arts
A conventional rewritable phase change type optical recording medium technique is as noted below.
These optical recording media have recording layers comprising tellurium and the like as main components. To record information, a laser pulse focused on the recording layer of crystallized state is irradiated for a short time to partially melt the recording layer. The molten portion is quenched by thermal diffusion and solidified to form an amorphous state recording mark. The light reflectivity of this recording mark is lower than that of the crystallized state and thus is optically reproducible as a recorded signal.
To erase information, the recording mark portion is irradiated with the laser beam and heated to a temperature lower than the melting point of the recording layer and higher than the crystallizing temperature. As a result the amorphous state recording mark is crystallized to return to the original non-recorded state.
As the materials for these rewritable phase change type optical recording media, there is known for example an alloy of Ge.sub.2 Sb.sub.2 Te.sub.5 or the like (N. Yamada et al., Proc. Int. Symp. on Optical Memory 1987 p61-66).
In these optical recording media having Te alloy as a recording layer, the crystallization speed is fast, and high speed overwriting by circular one-beam is feasible by only modulating the irradiation power. In the optical recording media using these recording layers, typically dielectric layers having heat resistant property and light penetrability are provided, one on each side of the recording layer, to prevent formation of deformations and openings in the recording layer at recording. Furthermore, it is known to provide a metal reflective layer, such as light reflective Al, by lamination on a dielectric layer opposite to the light beam incident direction. This is to improve the signal contrast in reproduction by the optical interference effect.
Difficulties arising with the conventional rewritable phase change type optical recording media as described above are as follows: The conventional disk structure has problems such as degradation of the recording wave forms in the write-start portion and write-end portion of the record in the recording region (hereinafter to be referred to as sector) or aggravation of jitter characteristics by repetition of the rewriting of records (In this regard, as is well known in this field, jitter characteristics concern deviations between a reproduced signal and an original signal for recording). Especially, the aforesaid problems become more significant when adopting a mark length recording which can be formed in high density in place of the conventional pit position recording. One of the causes of this problem may be, shifting of the recording layer to the track direction and radial direction by repetition of rewriting operations. That is, one of the causes is considered to be that, upon repetition of rewriting operations, the recording film gradually shifts forward and/or backward of the track, and the recording film gathers at the starting end and the terminating end of recording, or perhaps the film becomes thinner. Similarly, with respect to the radial direction, it is possible that the recording film at the central part in the track width direction is pushed aside to accumulate on the lateral wall of the track, and that the recording film at the central part in the widthwise direction of the track becomes thin. This is further considered to be a cause of the lowering of the tracking stability arising from repeated rewriting of records.
Furthermore, there has been the problem that, in case of the application of the land/groove recording (in which the higher density recording is feasible), the jitter of the land due to the repetition of rewriting is liable to be more aggravated than that of the groove.
The present invention seeks to provide a rewritable phase change type optical recording medium which shows lower degradation in the write-start portion and/or write-end portion of sectors. The present invention also aims to provide such an optical recording medium for which the characteristics are good.